Art of making concentric transmission lines



June 6, 1950 WOLF 2,510,358

ART OF MAKING CONCENTRIC TRANSMISSION LINES Filed March 20, 1946' IN! INTOR.

465561 J 1461/ BY @QM irrakwsy Patented June 1950 ART OF KING GONGENTBICTRANS- MISSION LINES Lester J. Well, Audubon,

Corporation oi America, ware N.J., assignmaeorporaflon to Radio of Dela-Application March 20, 1946, Serial No. 655,697 4 China. (Cl. 174-28)making electrical transmission lines, particularly concentrictransmission lines, and has special reference to the provision ofimprovements in so called button type insulators for maintaining theseparate conductors of such a line in spaced relation.

The prior art shows both elastic and non-elastic button' type insulatorsfor supporting an inner conductor in coaxial relation with respect to anouter conductor. Elastic buttons, constituted of paragutta or otherrubber-like materials, possess the advantage of being tight-fitting (inthat they are self-biased into engagement with the outer conductor) butare seldom used because such materials exhibit a higher power factor anda higher dielectric constant than is desirable where currents of radiofrequencies are involved. On the other hand, spacers constituted of manyof the common plastic or ceramic materials possess the requisiteinsulating properties but, because of their non-elastic nature, are nottight-iitting. This latter disadvantage is especial- 1y objectionablewhen the line is called upon to handle currents of substantial voltagesince, in this case, even a relatively minute air-space around theperiphery of the button (1. e. between the button and the inner surfaceof the outer conductor) augments the possibility of corona discharge.

Accordingly, the principal object of my present invention is to obviatethe foregoing and other less apparent objections to present daytransmission lines.

A related object of my invention is to provide a coaxial cable whichshall exhibit an extremely low attenuation factor for currents of radiofrequency, and one wherein the possibility of corona discharge isminimized.

Another and specific object of my invention is to provide an improvedbutton-type insulating spacer for the inner conductor of a coaxialcable, and one which is capable of being maintained in intimate,non-yielding contact with the inner surface of the outer conductor.

Still another object of my present invention is to provide a simple yetreliable method of manufacturing a concentric transmission line and onewhich in its practice ensures the correct permanet and rigid alignmentof the inner conductor with respect to the outer conductor.

Stated generally, my invention is predicated upon an appreciation of thefact, and its application to the art of making electrical transmissionlines, that certain non-elastic yet distortable 2 synthetic plasticmaterials, when distorted, exhibit a "plastic memory," i. e. theability, especially when heated, to return to the shape and dimensionswhich they possessed when cast, or prior to being distorted. Among thematerials possessing this property, as well as the insulating propertiesrequired for the purposes of this invention, are (1) polyethylene, (2)polytetrafluorethylene, (3) cross-linked polystyrene, (4) cross-linkedpolymethyl methacrylate resin (5) polyamide resins and (6) solidsilicones.

In carrying my invention into effect I either cast, or cut from a sheetof such material, a desired number of buttons each of a diameterslightly larger than the inner diameter oi the outer conductor of thetransmission line and string said buttons in spaced relation on the wireor tube which is to comprise the inner conductor of the line. Prior tothe "stringing operation I distort the said buttons to permit theassembly to be entered into the bore of the outer conductor and,finally, I heat the spacers, either by conduction or with an electricfield, to cause them to assume substantially the original shape anddimensions dictated by their plastic memory.

Other details of construction together with additional objects andadvantages will be apparent and my invention itself will be bestunderstood by reference to the following specification and to theaccompanying drawing, wherein:

Fig. l is a view in perspective of a button-type spacer which isconstituted, in accordance with my invention, of a distortableinsulating material, having a plastic memory;

Fig. 2 is a similar view of the button of Fig. 1, when distorted;

Fig. 3 is a broken-away view of a concentric line showing the manner inwhich the inner condoctor and its distorted spacers are assembled withinthe outer conductor;

Fig. 4 is a sectional view of a button distorted in an alternative formand Fig. 5 is a broken-away view of the transmission line of Fig. 3after the button-like spacers oi. Figs. 2 and 3 have been caused toassume the original shape and dimensions dictated by their plasticmemory.

In the drawing wherein I have used like reference characters todesignate the same or corresponding parts in all figures I designatesthe inner conductor, and 2 the outer conductor of a concentrictransmission line or cable. The dimensions of the cable which arepertinent to my invention are the inner diameter D of the outerconductor and the outer diameter d of the inner conductor. In thecompleted cable shown in Fig. the inner conductor i is supported withinthe outer conductor 2 by insulating button-like spacers 3 which areformed in accordance with my invention of a material (e. g.polyethylene) having a plastic memory.

As shown in Fig. 1 the button 3 in its original form, has parallel majorfaces a and b and has an outer diameter D slightly larger than the innerdiameter D of the outer conductor 3, and an inner diameter d slightlyless than the outer diameter d of the inner conductor i. As previouslyindicated this button 3 may conveniently be cut or punched from a thinsheet of material, though it may be formed by casting in a mould.Subsequently, preferably prior to their assembly upon the innerconductor, the buttons 3 are placed in a coining die or pressure mold(not shown) and forced to change their shape, for example in one of thepatterns shown in Figs. 2, and 3, or 4. In Figs. 2 and 3 the button hasbeen shaped by the die in the form of a convex cone and, in theembodiment of Fig. 4 comprises a hollow right circular cone wherein thethickness of its walls is greater adjacent the apex of the cone than itis at the base or lower perimeter. I prefer the latter form for highvoltage work since the thickness of the material is greatest adjacent tothe inner conductor where the lines of force of the field are the moreconcentrated. Irrespective of the exact shape for the button the coiningdie should operate to compress the blanks in such a way that the outerdiameter is made slightly less than the inner diameter D of the outerconductor and the inner diameter of central aperture is made slightlylarger than the outer diameter d of the inner conductor i which it isdesigned to accommodate.

When the insulating blanks of Fig. 1 have been distorted into one of theforms shown in Figs. 2 or- 4 they are assembled or strung in spacedrelation on the inner conductor 51 which is then placed, with thespacers properly positioned, within the outer conductor 2. As shown inFig. 3, at this time the insulators 3 are loose in the outer conductorand on the inner conductor. Thereafter the insulating buttons or,alternatively, the entire assembly is heated to a temperature (say 300C. or higher) sufficient to activate the plastic memory of theinsulating material so that the buttons are urged to change their shapein accordance with their past history. In the instant case the buttonstend to go back to the shape shown in Fig. 2. As a result the buttonsare urged into intimate contact with both the inner and outer conductorsl and 2, though because of their original oversizeddimensions they willnot quite achieve their original shape.

It will now be apparent that my invention atfords a simple yet reliablemethod of manufacturing a concentric transmission line having anextremely lowattenuation factor for currents of radio frequencies andone wherein the possibility of corona is minimized.

I claim:

1. A spacer for supporting an inner conductor within an outer conductor,said spacer comprising a button having a curved face of a diameter lessthan the inner diameter of said outer conductor and formed by distortinga blank of a relatively larger diameter constituted of a distortebleinsulating material of the type having a plastic memory.

2. The invention as set forth in claim 1 and wherein said buttoncomprises a cone having a concave face.

ing said spacers to permit said assembly to be' entered into said outerconductor, and then heating said spacers to cause them to assume theoriginal shape and dimensions dictated by their plastic memory.

LESTER J. WOLF.

REFERENCES omen The following references are of record in the file ofthis patent:

UNI'Im STATES PATENTS

